You didn't list choices, but the two possible answers would be:
<h2>Germany resumed unrestricted submarine warfare.</h2><h2>The Zimmermann Telegram revealed a German attempt to gain Mexico as an ally against the United States.</h2><h2 />
Context/detail:
Public outrage in the US against the Germans swept the nation following the sinking of the British ocean liner, Lusitania, which had happened in May, 1915. When a German U-boat (submarine) sank the Lusitania, over 1,000 persons were killed, including more than 100 Americans. The passenger liner was targeted by the Germans because they suspected weapons were being shipped to Britain in the cargo hold of the ship.
Germany managed to stave off American entry into the war at the time by pledging to stop submarine attacks. But two years later, the Germans resumed such attacks. This inflamed public opinion against Germany once again and poised America to go to war.
In addition, British intelligence intercepted and decoded a telegram (known as the "Zimmermann Telegram") that showed Germany was trying to secure Mexico as an ally against the United States. The revelation of this plan inflamed feelings in the United States. The telegram was sent in January, 1917. In March, 1917, German Foreign Secretary Arthur Zimmermann admitted the telegram was, in fact, sent by his office.
In April, 1917, the US declared war on Germany in response to the Germans' submarine warfare and the Zimmermann Telegram.
The Missouri compromise was nullified by the Kansas-Nebraska act. This act allowed the people in each state to determine whether or not slavery would continue within their state.
Abolitionist John Brown was executed because of his attempt to seize the arsenal at Harper's Ferry. John Brown led the Harper's Ferry raid.
